Gallium nitride crystal, group 13 nitride crystal, group 13 nitride crystal substrate, and manufacturing method

ABSTRACT

A gallium nitride crystal having a hexagonal crystal structure includes a first region located on an inner side of a cross section intersecting c-axis of the hexagonal crystal structure, and a second region surrounding at least a part of the outer periphery of the first region in the cross section. An emission spectrum of each of the first region and the second region with electron beam or ultraviolet light excitation has a first peak including a band edge emission of gallium nitride and a second peak located in a longer wavelength area than the first peak. A peak intensity of the first peak is smaller than a peak intensity of the second peak in the first region, and a peak intensity of the first peak is greater than a peak intensity of the second peak in the second region.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2011-201207 filed in Japan on Sep. 14, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gallium nitride crystal, a group 13 nitride crystal, a group 13 nitride crystal substrate, and a manufacturing method.

2. Description of the Related Art

Semiconductor materials based on gallium nitride (GaN) are used for blue color LED (light-emitting diode) or white color LED, and a semiconductor device such as semiconductor laser (also called “LD: Laser Diode”). The white color LED is used for illumination purpose or back lighting of cell phones, LC (Liquid Crystal) display or the like. The blue color LED is used for traffic lights or other illumination purpose and so on. On the other hand, blue-violet semiconductor laser is used as light sources of Blu-ray discs. Presently, most of semiconductor devices based on GaN used as light sources of UV or violet-blue-green color are produced by using MO-CVD (Metalorganic Chemical Vapor Deposition) method or MBE (Molecular Beam Epitaxy) method to grow crystal on sapphire or SiC substrate.

There are problems in a case that sapphire or SiC is used as substrate. Crystal defects increases because of the significant difference of thermal expansion coefficient or lattice constant between the substrate and the group 13 nitride. Such a defect affects device properties. For example, it becomes harder to elongate the lifetime of emitting device. And, the operating power may increase. In order to address these problems, it is the most preferable way to use a gallium nitride substrate which is made of the same material as crystal to be grown on the substrate.

Presently, free-standing GaN substrates are manufactured in such a manner that a thick gallium nitride crystal is grown on a hetero-substrate such as sapphire substrate or GaAs substrate by HVPE (Hydride Vapor Phase Epitaxy) with employing ELO (Epitaxial Lateral Overgrowth) which is a method to reduce the dislocation density, and then the thick film of gallium nitride is separated from the hetero-substrate. The gallium nitride substrate manufactured as such has a dislocation density reduced to the order of 10⁶ cm⁻², and allows a size up to 2 inches in practical use mainly for laser device purpose. Recently, there is a further need for much larger diameter of substrate up to 4 inches or 6 inches for electronic devices, or cost saving of white color LEDs.

Warpage or cracks which may be induced by the difference of the thermal expansion coefficient or the lattice constant between the hetero-substrate and the gallium nitride hinders to enlarge the diameter of substrate. The aforementioned dislocation density still remains. There is also a problem of high manufacturing cost in processes of separating one thick film of gallium nitride from one hetero-substrate, and polishing it to form the gallium nitride substrate.

On the other hand, as one of liquid phase methods to realize the gallium nitride substrate, many efforts have been made for developing a flux method in which the gallium nitride crystal is formed by dissolving the nitrogen from a gaseous phase into a molten mixture of group 13 metal and alkali metal.

In the flux method, a molten mixture containing the alkali metal such as sodium (Na) and potassium (K) and the group 13 metal such as gallium (Ga) is heated to about 600 to 900 degrees Celsius under an atmosphere where the nitrogen pressure is 10 MPa or less. Thus, the nitrogen is dissolved from the gaseous phase and reacts with the group 13 metal in the molten mixture to form the group 13 nitride crystal. The flux method allows a crystal growth with a lower temperature and lower pressure in comparison with other liquid phase methods. The crystal formed by the flux method has a low dislocation density advantageously lower than 10⁶ cm⁻².

There is a report that gallium nitride crystal is formed under conditions that sodium azide (NaN₃) and metal Ga which are used as source materials are put and sealed in a reactor vessel made of stainless material (as for sizes inside of the vessel, inner diameter is 7.5 mm, length is 100 mm) under a nitrogen atmosphere, and the reactor vessel is retained at 600 to 800 degrees Celsius for 24 to 100 hours (Chemistry of Materials Vol. 9 (1997) 413-416).

Japanese Patent Application Laid-open No. 2008-94704 discloses a method of manufacturing a column-like crystal of gallium nitride by using a needle-like crystal of aluminum nitride (AlN) as seed crystal in order to provide a large crystal of gallium nitride. Japanese Patent Application Laid-open No. 2006-045047 discloses a method of manufacturing a needle-like crystal of aluminum nitride which becomes a seed crystal. Japanese Patent Application Laid-open No. 2009-126771 discloses a seed crystal of which a yellow emission effect is observed, and a gallium nitride crystal which is formed on the seed crystal and has a crystal layer of which a yellow emission effect is not observed.

In a case that gallium nitride crystal is grown from aluminum nitride as seed crystal, however, the difference of lattice constant between aluminum nitride and gallium nitride may cause dislocations due to the lattice mismatch. Since thermal expansion coefficient is also different between aluminum nitride and gallium nitride, thermal stress may cause new dislocations or even cracks, in the course of cooling from a crystal growth temperature to a room temperature.

Therefore, it is preferable to use gallium nitride crystal as seed crystal which has the same lattice constant or the same thermal expansion coefficient with the target crystal, in order to grow a high quality gallium nitride crystal with low dislocation density. However, it is difficult to grow a needle-like crystal of gallium nitride by the method disclosed in Japanese Patent Application Laid-open No. 2006-045047.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve the problems in the conventional technology.

A gallium nitride crystal having a hexagonal crystal structure includes a first region located on an inner side of a cross section intersecting c-axis of the hexagonal crystal structure, and a second region surrounding at least a part of the outer periphery of the first region in the cross section. An emission spectrum of each of the first region and the second region with electron beam or ultraviolet light excitation has a first peak including a hand edge emission of gallium nitride and a second peak located in a longer wavelength area than the first peak. A peak intensity of the first peak is smaller than a peak intensity of the second peak in the first region, and a peak intensity of the first peak is greater than a peak intensity of the second peak in the second region.

A group 13 nitride crystal contains therein at least a part of the aforementioned gallium nitride crystal.

A group 13 nitride crystal substrate contains at least a part of the aforementioned gallium nitride crystal.

A method of manufacturing a gallium nitride crystal includes a first process to grow a first region of the gallium nitride crystal containing boron, and a second process to grow a second region of the gallium nitride crystal outside of the first region, the second region having a boron density lower than the boron density of the first region.

A method of manufacturing a group 13 nitride crystal is a method in which the aforementioned gallium nitride crystal is used as a seed crystal to grow the group 13 nitride crystal.

The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section illustrating a crystal manufacturing apparatus to manufacture a seed crystal in an embodiment;

FIG. 2 is a schematic cross section illustrating a structure of a gallium nitride crystal according to the embodiment;

FIG. 3 is a view illustrating an example of c-plane cross section of a gallium nitride crystal according to an embodiment;

FIG. 4 is a view illustrating another example of c-plane cross section of the gallium nitride crystal;

FIG. 5 is a graph of exemplary spectra of a first region and a second region excited with electron beam or ultraviolet light;

FIG. 6 is a view illustrating another example of c-plane cross section of the gallium nitride crystal;

FIG. 7 is a schematic cross section illustrating an exemplary configuration of a crystal manufacturing apparatus used to grow a bulk crystal from a seed crystal;

FIG. 8 is a perspective view illustrating an example of group 13 nitride crystal;

FIG. 9 is a perspective view illustrating an example of group 13 nitride crystal;

FIG. 10 is a perspective view illustrating an example of group 13 nitride crystal;

FIG. 11 is a perspective view illustrating an example of group 13 nitride crystal;

FIG. 12 is a schematic view illustrating dislocations in a cross section parallel to c-axis and a-axis;

FIG. 13 is a schematic view illustrating directions to slice a group 13 nitride crystal;

FIG. 14 is a schematic view illustrating directions to slice a group 13 nitride crystal;

FIG. 15A is a schematic view illustrating an example of crystal substrate;

FIG. 15B is a schematic view illustrating an example of crystal substrate;

FIG. 15C is a schematic view illustrating an example of crystal substrate;

FIG. 16A is a schematic view illustrating an example of crystal substrate;

FIG. 16B is a schematic view illustrating an example of crystal substrate;

FIG. 16C is a schematic view illustrating an example of crystal substrate;

FIG. 17 is a schematic view illustrating a crystal growth process of group 13 nitride crystal from seed crystal;

FIG. 18 is a schematic view illustrating a crystal growth process of group 13 nitride crystal from seed crystal;

FIG. 19 is a schematic view illustrating a crystal growth process of group 13 nitride crystal from seed crystal;

FIG. 20 is a schematic view illustrating various relationships between seed crystal shape and L/d;

FIG. 21 is a mapping image of spectral intensity from 360 nm to 370 nm in photoluminescence;

FIG. 22 is a mapping image of spectral intensity from 500 nm to 800 nm in photoluminescence; and

FIG. 23 is a schematic view illustrating an emission distribution in photoluminescence of the crystal substrate 100 according to Example 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now an explanation will be made on a gallium nitride crystal, a group 13 nitride crystal, and a group 13 nitride crystal substrate, as well as a manufacturing method of them, with reference to the accompanying drawings. The accompanying drawings herein only schematically show shapes, sizes and arrangements of constituents, elements or components to an extent necessary for a clear understanding of the present invention. However, these drawings shall not be considered any limitation to the present invention. Identical reference numerals are given to the same or corresponding constituents, elements or components among the plurality of drawings, while the redundant explanation may be omitted as appropriate.

[1] Crystal Manufacturing Method of Seed Crystal <Crystal Manufacturing Apparatus>

FIG. 1 schematically shows a crystal manufacturing apparatus 1 to produce a seed crystal 25 according to an embodiment. The crystal manufacturing apparatus 1 has a double structure in which an inner vessel 11 is disposed in an outside pressure resistant vessel 28 made of stainless material, and a reactor vessel 12 is disposed in the inner vessel 11. The inner vessel 11 is attachable to and detachable from the outside pressure resistant vessel 28.

The reactor vessel 12 is for maintaining a molten mixture 24 obtained by melting source material(s) and/or additive(s) in order to grow the seed crystal 25. The configuration of the reactor vessel 12 will be described later.

To the outside pressure resistant vessel 28 and the inner vessel 11, gas supplying pipes 15 and 32 are respectively connected to supply nitrogen (N₂) gas which is source material of the group 13 nitride crystal and a diluent gas for controlling a whole pressure to an inner space 33 of the outside vessel 28 and an inner space 23 of the inner vessel 11. A gas supplying pipe 14 is branched into a nitrogen supplying pipe 17 and a diluent gas supplying pipe 20 via valves 15 and 18, respectively. These pipes 17 and 20 can be separated at valves 15 and 18, respectively.

Argon (Ar) gas which is inert gas is preferably used as the diluent gas. However, the embodiment is not limited to this. For example, other inert gas such as helium (He) may be used as the diluent gas.

The nitrogen gas is introduced from the nitrogen supplying pipe 17 which is connected to a gas canister or the like containing the nitrogen gas therein. The nitrogen gas from the pipe 17 is subjected to a pressure control by a pressure regulator 16 and then supplied to the gas supplying pipe 14 via the valve 15. On the other hand, the diluent as (e.g. Ar gas) is introduced from the diluent gas supplying pipe 20 which is connected to a gas canister or the like containing the diluent gas therein. The diluent gas from the pipe 20 is subjected to a pressure control by a pressure regulator 19 and then supplied to the gas supplying pipe 14 via the valve 18. Thus, the pressure regulated nitrogen gas and the pressure regulated diluent gas are supplied to the gas pipe 14 to be mixed there.

The gas mixture of the nitrogen gas and the diluent gas is supplied to the outside vessel 28 and the inner vessel 11 from the pipe 14 via the valves 15 and 32. The inner vessel 11 can be removed from the apparatus 1 at the valve 29.

A pressure meter 22 is disposed on the pipe 14 for monitoring a whole pressure inside of the outside vessel 28 and the inner vessel 11 and controlling pressures inside of the outside vessel 28 and inner vessel 11.

In the embodiment, nitrogen partial pressure can be controlled by controlling the pressures of the nitrogen gas and the diluent gas by means of valves 15 and 18 and the pressure regulators 16 and 19. Since the whole pressure of the outside vessel 28 and the inner vessel 11 can be also controlled, it is possible to reduce evaporation of alkali metal (e.g. sodium) in the reactor vessel 12 by increasing the whole pressure of the inner vessel 11. In other words, it is possible to control separately the nitrogen partial pressure which becomes a nitrogen source affecting the crystal growth conditions of GaN and the whole pressure which affects the reduction of sodium evaporation.

As shown in FIG. 1, a heater 13 is disposed around the outer circumference of the inner vessel 11 inside of the outside vessel 28 for heating the inner vessel 11 and the vessel 12 and controlling the temperature of the molten mixture 24.

<Manufacturing Method of Crystal>

The manufacturing method according to the embodiment is a method of producing a seed crystal 25 of gallium nitride by the flux method. Incidentally, the seed crystal 25 may be simply referred to as gallium nitride crystal 25.

The manufacturing method of crystal according to the embodiment includes the first process to grow the first region of gallium nitride crystal containing boron, and the second process to grow, outside of the first region, the second region of gallium nitride crystal having the boron density lower than that of the first region.

In order to grow the crystal with different boron densities between the inner side and the outer side of the gallium nitride crystal 25, the manufacturing method of crystal according to the embodiment includes a boron melting process to melt the boron into the molten mixture 24, a boron taking process to take the boron into the crystal during the growth of the gallium nitride crystal 25, and a boron reducing process to reduce the boron density in the molten mixture 24 in the course of the crystal growth.

In the boron melting process, the boron is melted into the molten mixture 24 from boron nitride (BN) contained in the inner wall of the reactor vessel 12 or from a boron nitride material disposed inside of the reactor vessel 12. Then, a small amount of the melted boron is taken into the crystal while the gallium nitride crystal 25 grows (the boron taking process). An amount of boron taken into the gallium nitride crystal 25 is gradually reduced as the crystal grows (the boron reducing process).

Due to the boron reducing process, it is possible to reduce the boron density of the outer region in a cross section intersecting the c-axis in comparison with the boron density of the inner region in the cross section, while the gallium nitride crystal 25 grows with growing the m-planes ({10-10} planes). Thereby, the density of boron as an impurity can be reduced and the dislocation density which may be induced by the impurities can be reduced, at the outer circumference surface comprised of the m-planes (six side surfaces of the hexagonal column) of the gallium nitride crystal 25. Thus, the outer circumference of the gallium nitride crystal 25 can be made of the higher quality crystal in comparison with the inner region.

In a manufacturing method as described in the following [3] to grow the group 13 nitride crystal 27 from the seed crystal 25 to form the group 13 nitride crystals 80 to 83, the group 13 nitride crystal 27 is grown mainly from side surfaces of the seed crystal 25 (the outer circumference surface comprised of m-planes) as start points of the crystal growth. Thereby, if the outer circumference surface comprised of m-planes of the seed crystal 25 is of a good quality as described above, the crystal 27 grown from the surface becomes of a good quality. Therefore, according to the embodiment, it is possible to grow a large and high quality seed crystal 25 and thereby a high quality group 13 nitride crystal 27.

Next, a further detail explanation will be made on the boron melting process, the boron taking process and the boron reducing process.

(1) Method That Reactor Vessel 12 Contains Boron Nitride

In an example of the boron melting process, a reactor vessel 12 made of a sintered BN (sintered boron nitride) can be used as the reactor vessel 12. In the course of heating the reactor vessel 12 to a crystal growth temperature, boron is melted from the reactor vessel 12 into the molten mixture 24 (the boron melting process). Then, the boron in the molten mixture 24 is taken into the seed crystal 25 in the course of growing the seed crystal 25 the boron taking process). The boron in the molten mixture is gradually reduced as the seed crystal 25 grows (the boron reducing process).

In the aforementioned example, the reactor vessel 12 made of the sintered BN is used. However, the reactor vessel 12 is not limited to this. In a preferred embodiment, it is sufficient that the reactor vessel 12 includes a BN-contained material (e.g. sintered BN) at a part at least of its inner wall coming into contact with the molten mixture 24. The other part of the reactor vessel 12 may be made of nitride such as pyrolytic BN (P—BN) and the like, oxide such as alumina, YAG and the like, carbide such as SiC and the like.

(2) Method That BN-Contained Material is Placed in the Reactor Vessel 12

In another example of the boron melting process, a BN-contained material may be placed in the reactor vessel 12. For example, a sintered BN may be placed in the reactor vessel 12. Materials for the reactor vessel 12 are not limited to any particular one similarly to the process (1).

In this method, in the course of heating the reactor vessel 12 to a crystal growth temperature, boron is gradually melted from the material placed in the reactor vessel 12 into the molten mixture 24 (the boron melting process).

In the processes (1) and (2), crystal nuclei of GaN crystal 25 are likely to be formed on a surface of the BN-contained material coming into contact with the molten mixture 24. Therefore, if the BN surface (i.e. the inner wall surface or the material surface) is gradually covered with the GaN crystal nuclei formed thereon, the amount of boron melted from the covered BN material into the molten mixture 24 gradually decreases (the boron reducing process). Furthermore, in accordance with the growth of the GaN crystal, a surface area of the crystal gradually increases and thereby the density of boron which is taken into the GaN crystal decreases 25 (the boron reducing process).

In the processes (1) and (2), the boron-contained material is used for melting the boron into the molten mixture 24. A method or process for melting the boron into the molten mixture 24 is not limited to this. For example, boron may be added into the molten mixture 24, or any other process may be used. As for a method or process for reducing the boron density in the molten mixture 24, any other method or process may be used. The crystal manufacturing method according to the embodiment sufficiently includes the boron melting process, the boron taking process and the boron reducing process, as mentioned above at least.

<Preparation of Materials or the Like, and Crystal Growth Conditions>

An operation to put the source material or the like into the reactor vessel 12 is conducted in such a manner that the inner vessel 11 is put into a glove box under an inert gas atmosphere such as Ar.

In a case that the seed crystal 25 is grown by the method (1), a material used as a flux, and source material(s) are put into the reactor vessel 12 as explained in the method (1).

In a case that the seed crystal 25 is grown by the method (2), a BIT-contained material as explained in the method (2), a material used as a flux and source material(s) are put into the reactor vessel 12 as explained in the method (2).

The material used as flux may be sodium, or sodium compound (e.g. sodium azide). Any other material including other alkali metals such as lithium and potassium, or compounds of these alkali metals may be used as the flux. Furthermore, alkali earth metals such as barium, strontium, and magnesium, or compounds of these alkali earth metals may be used as the flux. A plurality kind of alkali metals or alkali earth metals may be used.

As the source material, gallium is used. Any other materials including without limitation other group 13 elements such as aluminum and indium, or mixtures thereof may be used as source material(s) to be put into the reactor vessel 12.

After setting the source material(s) and others as mentioned above, the inner vessel 11 and the reactor vessel 12 inside of the inner vessel 11 are heated to a crystal growth temperature by turning on the heater 13. Then, the source material is melted with the material used as the flux in the reactor vessel 12 to form the molten mixture 24. Nitrogen as the source material of the seed crystal 25 can be supplied to the molten mixture 24 by bring the nitrogen having the aforementioned partial pressure into contact with the molten mixture 24 and thereby dissolving the nitrogen into the molten mixture 24. Furthermore, boron is melted into the molten mixture 24 as described above (the boron melting process and the molten mixture forming process).

Crystal nuclei of gallium nitride crystal 25 are formed on the inner wall of the reactor vessel 12 from source materials melted in the molten mixture 24. The source materials and boron in the molten mixture 24 are supplied to these nuclei to grow the nuclei to the needle-like gallium nitride crystal 25. In the course of the crystal growth of the gallium nitride crystal 25, as described above, boron in the molten mixture 24 is taken into the gallium nitride crystal 25 (the boron taking process), so that the “boron-rich” first region 25 a (FIG. 2) is likely to be formed on the inner side of the gallium nitride crystal 25. Then, as the crystal grows, the amount of boron taken into the crystal reduces (the boron reducing process). Thus, the “boron-poor” second region 25 b (FIG. 2) is likely to be formed on the outside of the first region 25 a.

Preferably, in the crystal manufacturing method according to the embodiment, the partial pressure of nitrogen in the pressure resistant vessel 11 is 5 MPa to 10 MPa.

Preferably, in the crystal manufacturing method according to the embodiment, the temperature in the molten mixture 24 (the crystal growth temperature) is 800 degrees Celsius to 900 degrees Celsius.

In a preferred embodiment, a mol ratio of alkali metal to the total mol number of gallium and alkali metal (e.g. sodium) is 75% to 90%, the crystal growth temperature of the molten mixture 24 is 860 degrees Celsius to 900 degrees Celsius, and the partial pressure of nitrogen is 5 MPa to 8 MPa.

In a further preferred embodiment, the mol ratio of gallium to alkali metal is 0.25:0.75, the crystal growth temperature is 860 degrees Celsius to 870 degrees Celsius, and the partial pressure of nitrogen is 7 MPa to 8 MPa (see Examples).

[2] Seed Crystal

The gallium nitride crystal according to the embodiment is a seed crystal 25 manufactured by the method mentioned in [1].

<Boron Density Property>

FIG. 2 schematically shows a cross section of gallium nitride crystal according to the embodiment. Incidentally, FIG. 2 illustrates the cross section parallel to the c-axis and the a-axis in the gallium nitride crystal 25 having a hexagonal crystal structure. In a preferred embodiment, the gallium nitride crystal 25 is elongated in the c-axis direction as illustrated in FIG. 2, and has a “boron rich” region (the first region 25 a) on inner side thereof and a “boron poor” region (the second region 25 b) on outer side thereof.

FIG. 3 and FIG. 4 show examples of the c-plane ({0001} plane) cross section of the gallium nitride crystal 25. Namely, FIG. 3 and FIG. 4 are cross sections along A-A line in FIG. 2. As illustrated in FIG. 3, the cross section orthogonal to the c-axis (c-plane) in the gallium nitride crystal 25 is a hexagon or almost hexagon. The side surfaces of the gallium nitride crystal 25 corresponding to edges of the hexagon are comprised mainly of m-planes of the hexagonal crystal structure.

As illustrated in FIG. 3, in a preferred embodiment, the gallium nitride crystal 25 has preferably the boron rich region (the first region 25 a) on inner side thereof in the c-plane cross section, and the boron poor region (the second region 25 b) preferably surrounds at least a part of the first region 25 a in the c-plane cross section. Thus, in a case that the gallium nitride crystal 25 in which the second region 25 b surrounds at least a part of the outer periphery of the first region 25 a is used as seed crystal, and the group 13 nitride crystal 27 (FIG. 8) is grown from the outer periphery of the seed crystal, the group 13 nitride crystal 27 grown from the second region 25 b is likely to be of high quality.

Incidentally, the first region 25 a and the second region 25 b are included in the c-plane cross section. However, the embodiment is not limited to the precise c-plane cross section. Thus, it is sufficient that the first region 25 a and the second region 25 b are included in at least a cross section intersecting the c-axis.

In the gallium nitride crystal 25 of a preferred embodiment, the boron density of the first region 25 a on the inner side is preferably 4×10¹⁸ atms/cm³ or more, the boron density of the second region 25 b on the outer side is preferably less than 4×10¹⁸ atms/cm³ (see Examples 1, 2).

In a further preferred embodiment, the second region 25 b on the outer side preferably surrounds an entire outer periphery (i.e. all outer circumferential surfaces of the hexagonal shape comprised of generally m-planes) of the first region 25 a on the inner side in a cross section intersecting the c-axis of the gallium nitride crystal 25 (FIG. 4).

FIG. 4 shows another example of the c-plane cross section of the gallium nitride crystal 25. As illustrated in FIG. 4, in a case that the second region 25 b surrounds an entire periphery of the first region 25 a comprised of m-planes, the group 13 nitride crystal 27 can be grown from the second region 25 b, and thereby the higher quality region can be increased in the group 13 nitride crystal 27.

In the gallium nitride crystal 25 of a preferred embodiment, the thickness “t” (FIG. 2) of the second region 25 b on outer side is preferably 100 nm or more.

In a case that the group 13 nitride crystal is grown from GaN crystal as a seed crystal by a flux method, as described in Japanese Patent Application Laid-open No. 2009-126771, a melt-back of the seed crystal may arise. It is known that an amount of melt-back (melt-back amount) increases when the seed crystal is of low quality, especially the seed crystal includes an affected or damaged layer remained therein.

To the contrary, the gallium nitride crystal 25 according to the embodiment has 100 nm or more thickness of the second region 25 b which is a high quality crystal layer on the cuter side of the crystal. Thereby, even in a case that the melt-back arises in the course of growing the seed crystal 25, the second region 25 b is likely to be remained. Therefore, the group 13 nitride crystal 27 of high quality can be easily formed.

<Emission Property>

FIG. 5 shows an example of emission spectra with electron beam or UV excitation of the first region 25 a and the second region 25 b (see Examples 1, 2).

The gallium nitride crystal 25 according to the embodiment includes the first region 25 a on an inner side of the cross section intersecting the c-axis of the gallium nitride crystal having the hexagonal crystal structure, and includes the second region 25 b surrounding at least a part of the first region 25 a in the cross section. In emission spectrum of the first region 25 a with electron beam or UV excitation, the first peak including the band edge emission of GaN has the peak intensity smaller than that of the second peak which appears in longer wavelength area than the first peak. In emission spectrum of the second region 25 b with electron beam or UV excitation, the first peak has the peak intensity greater than that of the second peak.

The first peak refers to a peak which includes the band edge emission of GaN and which appears around about 365 nm in the measurement at a room temperature. The band edge emission of GaN is an emission due to a recombination of holes in an upper edge of the valence band and electrons in bottom of the conduction band in the gallium nitride crystal 25, and means that the light having energy (wavelength) equal to the band gap is emitted. In other words, the first peak is a peak due to a periodic structure of the crystal and a combination (status) of nitrogen and gallium in the gallium nitride crystal 25. The first peak may include an emission in the vicinity of the band edge in addition to the band edge emission.

The second peak refers to at least one peak which appears in a longer wavelength area than the first peak, refers to a peak of emission spectrum due to reasons other than the band edge emission or the emission in the vicinity of the band edge of GaN, and refers to a peak due to reasons other than the combination of nitrogen and gallium. For example, the second peak is a peak including an emission due to impurities or defects.

In a more preferred embodiment, the second peak appears in a range from 450 nm to 650 nm with respect to an emission spectrum with electron beam or UV excitation when measured at a room temperature.

In a still more preferred embodiment, the second peak appears in a range from 590 nm to 650 nm with respect to an emission spectrum with electron beam or UV excitation when measured at a room temperature.

The room temperature refers to about 20 degrees Celsius, and refers to a range of about 10 degrees Celsius to 30 degrees Celsius. When the emission spectra with electron beam or UV excitation was measured at a temperature lower than the room temperature (e.g. −270 degrees Celsius), the second peak may split into a plurality of peaks. The second peak according to the embodiment may split into a plurality peak when measured at a temperature other than the room temperature, insofar as the second peak appears as a broad peak when measured at the room temperature.

The emission spectra with electron beam or UV excitation can be obtained by measuring PL (Photoluminescence) using He—Cd laser (helium-cadmium laser) as exciting light source. However, the embodiment is not limited to this. It is possible to observe colors or intensities of spectra by using fluorescence microscope. The first region 25 a and the second region 25 b can be distinguished or identified by observed colors.

A fact that the second peak has a peak intensity greater than that of the first peak as shown in the emission spectrum of the first region 25 a means that the first region 25 a contains a relatively large amount of impurities or defects. On the other hand, a fact that the first peak has a peak intensity greater than that of the second peak as shown in the emission spectrum of the second region 25 b means that the second region 25 b contains less impurities or defects and thereby the second region 25 b is a high quality crystal.

The peak intensity of the first peak and the peak intensity of the second peak with respect to the first region 25 a and second region 25 b are not limited to any particular one, insofar as the aforementioned relationships are satisfied. In a preferred embodiment, the peak intensity of the first peak of the second region 25 b is greater than the peak intensity of the first peak of the first region 25 a. And, the peak intensity of the second peak of the second region 25 b is smaller than the peak intensity of the second peak of the first region 25 a.

Thus, in the embodiment, the second region 25 b having less impurities or defects is located on an outer side in the cross section intersecting the c-axis of the gallium nitride crystal 25. Thereby, it is possible to manufacture the high quality group 13 nitride crystals 80 to 83, in a case that the group 13 nitride crystal 27 is grown from the gallium nitride crystal 25 as seed crystal.

The gallium nitride crystal 25 is not limited to the double-layered structure made of the first region 25 a and the second region 25 b. The gallium nitride crystal 25 may have a multi-layered structure more than double-layered (e.g. three-layered). Specifically, the gallium nitride crystal 25 may include other regions different from the first region 25 a and the second region 25 b in its structures or properties. Alternatively, the gallium nitride crystal 25 may include a plurality of first regions or second regions. Alternatively, the gallium nitride crystal 25 may include a region whose emission spectrum with electron beam or UV excitation has a weak intensity.

FIG. 6 schematically shows another example of the c-plane cross section of the gallium nitride crystal 25. As shown in FIG. 6, the second region 25 b may be made of the second region 25 b 1 and the second region 25 b 2. In this case, the first peak of the second region 25 b 2 has a peak intensity smaller than that of the first peak of the second region 25 b 1. The second region 25 b 2 may be formed so as to surround the first region 25 a or the second region 25 b 1.

<Other Structures>

Additives may be doped to the gallium nitride crystal 25 (the first region 25 a and the second region 25 b) according to the embodiment. Additives may be a doping impurity such as germanium (Ge), oxygen (O), silicon Si) and the like, an acceptor impurity such as magnesium (Mg), lithium (Li), strontium (Sr), calcium (Ca), zinc (Zn) and so on, a transition metal for providing magnetic property such as iron (Fe), manganese (Mn), chromium (Cr) and the like, a rare earth element or the like for providing magnetic property or fluorescence such as europium (Eu), erbium (Er), terbium (Tb), thulium (Tm) and the like, congeners such as aluminum (Al), indium (In) and the like. Other additives may de selected and doped depending on the purpose.

In FIG. 3 and FIG. 4, the c-plane cross section of the gallium nitride crystal 25, the c-plane cross section of the first region 25 a, and the c-plane cross section of the second region 25 b are illustrated as regular hexagon, respectively. However, these drawings are only for illustrating examples. The c-plane cross section is not limited to the regular hexagon. The c-plane cross section of the gallium nitride crystal 25, the c-plane cross section of the first region 25 a, and the c-plane cross section of the second region 25 b are formed in generally hexagon due to the cross section of the gallium nitride crystal having a hexagonal crystal structure. In a case that other structure arises inside or at a boundary of these cross sections in the course of the crystal growth, the outlive of each hexagon may deform at the boundary with other structure.

[3] Manufacturing Method of Bulk Crystal from Seed Crystal

The manufacturing method of crystal according to the embodiment is a method of manufacturing a group 13 nitride crystal (e.g. gallium nitride crystal) by a flux method in which the gallium nitride crystal 25 mentioned in [2] is used as a seed crystal (seed crystal 25) to grow the group 13 nitride crystal.

<Crystal Manufacturing Apparatus>

FIG. 7 schematically shows an exemplary configuration of a crystal manufacturing apparatus 2 to grow the seed crystal 25. The apparatus 2 has a double vessel structure in which an inner vessel 51 is disposed in an outside pressure resistant vessel 50 made of stainless material, and a reactor vessel 52 is disposed in the inner vessel 51. The inner vessel 51 is attachable to and detachable from the outside pressure resistant vessel 50.

The reactor vessel 52 is for maintaining the seed crystal 25 with a molten mixture 24 of alkali metal and group 13 element-contained material in order to grow the seed crystal 25 (to grow the bulk crystal from seed crystal is referred to as SG or Seed Growth).

Materials for the reactor vessel 52 are not limited to any particular material. For example, nitrides such as sintered BN, P—BN and the like, oxides such as aluminum, YAG and the like, carbides such as SiC and the like, and so on may be used. An inner well of the reactor vessel 52, that is, a portion or portions of the reactor vessel 52 coming into contact with the molten mixture 24 is/are preferably made of material(s) insensitive to the molten mixture 24. As examples of materials allowing the crystal growth of gallium nitride, there may be nitrides such as boron nitride (BN), pyrolytic BN (P—BN), aluminum nitride and the like, oxides such as alumina, yttrium-aluminum-garnet (YAG) and the like, stainless steel (SUS) and so on.

To the outside pressure resistant vessel 50 and the inner vessel 51, gas pipes 65 and 66 are respectively connected to supply nitrogen (N₂) gas which is source material of the group 13 nitride crystal and a diluent gas for controlling a whole pressure to an inner space 67 of the outside vessel 50 and an inner space 68 of the inner vessel 51. A gas supplying pipe 54 is branched into a nitrogen supplying pipe 57 and a diluent gas supplying pipe 60 via valves 55 and 58, respectively. These pipes 57 and 60 can be separated at valves 55 and 58, respectively.

Argon (Ar) gas which is inert gas is preferably used as the diluent gas. However, the embodiment is not limited to this. For example, other inert gas such as helium (He) may be used as the diluent gas.

The nitrogen gas is introduced from the nitrogen supplying pipe 57 which is connected to a gas canister or the like containing the nitrogen gas therein. The nitrogen gas from the pipe 57 is subjected to a pressure control by a pressure regulator 56 and then supplied to the gas supplying pipe 54 via the valve 55. On the other hand, the diluent gas (e.g. Ar gas) is introduced from the diluent gas supplying pipe 60 which is connected to a gas canister or the like containing the diluent gas therein. The diluent gas from the pipe 60 is subjected to a pressure control by a pressure regulator 59 and then supplied to the gas supplying pipe 54 via the valve 58. Thus, the pressure regulated nitrogen gas and the pressure regulated diluent gas are supplied to the gas pipe 54 to be mixed there.

The gas mixture of the nitrogen gas and the diluent gas is supplied to the outside vessel 50 via the pipe 65 and the inner vessel 51 via the pipe 66. The inner vessel 51 can be removed from the apparatus 2 at the valve 61.

A pressure meter 64 is disposed on the pipe 54 for monitoring a whole pressure inside of the outside vessel 50 and the inner vessel 51 and controlling pressures inside of the outside vessel 50 and inner vessel 51.

In the embodiment, nitrogen partial pressure can be controlled by controlling the pressures of the nitrogen gas and the diluent gas by means of valves 55 and 58 and the pressure regulators 56 and 59. Since the whole pressure of the outside vessel 50 and the inner vessel 51 can be controlled, it is possible to reduce evaporation of alkali metal (e.g. sodium) in the reactor vessel 52 by increasing the whole pressure of the inner vessel 51. In other words, it is possible to control separately the nitrogen partial pressure which becomes a nitrogen source affecting the crystal growth conditions of GaN and the whole pressure which affects the reduction of sodium evaporation.

As shown in FIG. 7, a heater 53 is disposed around the outer circumference of the inner vessel 51 inside of the outside vessel 50 for heating the inner vessel 51 and the reactor vessel 52 and controlling the temperature of the molten mixture 24.

<Preparation of Source Materials or the Like and Crystal Growth Conditions>

An operation to put the seed crystal 25, the source material, the flux material, additives and the like such as gallium (Ga), sodium (Na), and carbon (C) as a dopant and so on into the reactor vessel 52 is conducted in such a manner that the inner vessel 51 is put into a glove box under an inert gas atmosphere such as Ar. Incidentally, this operation may be conducted in a state that the reactor vessel 52 is put in the inner vessel 51.

In the reactor vessel 52, the seed crystal 25 mentioned in [2] is installed. Into the reactor vessel 52, a material containing at least a group 13 element (e.g. gallium), and a material to he used as flux are also put.

Material used as flux may be sodium or compound thereof (e.g. sodium azide). Other alkali metals such as lithium, potassium and the like, or compounds of these alkali metals may be used as flux. Alkali earth metals such as barium, strontium, magnesium and the like, or compounds of these alkali earth metals may be used as flux. Plurality kinds of alkali metals or alkali earth metals may be used.

The material containing at least group 13 element as source material may be gallium which is one of group 13 elements. Any other materials including without limitation other group 13 elements such as boron, aluminum and indium, or mixtures thereof may be used as source material(s).

A mol ratio of the material containing or including the group 13 element to alkali metal is not limited to any particular one. However, the mol ratio of alkali metal to the total mol number of the alkali metal and the group 13 element is preferably 40% to 95%.

After the materials are set as mentioned above, the heater 53 is turned on to heat the inner vessel 51 and the reactor vessel 52 inside of the inner vessel 51 to a crystal growth temperature. Then, the material containing at least a group 13 element as source material, the alkali metal, other additives and the like are melted in the reactor vessel 52 to form the molten mixture 24. By bring the nitrogen having the aforementioned partial pressure into contact with the molten mixture 24 and thereby dissolving the nitrogen into the molten mixture 24, the nitrogen which is a source material of the group 13 nitride crystal 27 can he supplied to the molten mixture 24 (the molten mixture forming process).

Source materials melted in the molten mixture 24 are supplied onto the outer circumference surface of the seed crystal 25, so that the group 13 nitride crystal 27 is grown from the outer circumference surface of the seed crystal 25 with the source materials. The source materials are further supplied onto the outer peripheral surface of the group 13 nitride crystal 27 to promote the crystal growth of the group 13 nitride crystal 27. Thus, by growing the group 13 nitride crystal 27 from the seed crystal 25, the group 13 nitride crystals 80 (FIG. 8), 81 (FIG. 9), 82 (FIG. 10) and 83 (FIG. 11) each including the seed crystal 25 can be obtained (the crystal growth process).

In a preferred embodiment, the partial pressure of nitrogen gas in the inner space 68 of the inner vessel 51 and the inner space 67 of the outer pressure resistant vessel 50 is at least 0.1 MPa or more. In a more preferred embodiment, the partial pressure of nitrogen gas in the inner space 68 of the inner vessel 51 and the inner space 67 of the outer pressure resistant vessel 50 is in a range of 2 MPa to 5 MPa.

In a preferred embodiment, the temperature of the molten mixture 24 (the crystal growth temperature) is at least 700 degrees Celsius or more. In a more preferred embodiment, the crystal growth temperature is in a range of 850 degrees Celsius to 900 degrees Celsius.

In a case that the group 13 nitride crystal 27 is grown from the seed crystal 25, the dislocation density of the group 13 nitride crystal 27 grown mainly from the outer periphery comprised of m-planes of the seed crystal 25 is considered to be affected by the quality of the outer periphery comprised of m-planes of the seed crystal 25. As aforementioned in [2], the outer periphery comprised of m-planes of the seed crystal 25 is of high quality. Thereby, by growing the group 13 nitride crystal 27 from the seed crystal 25, it is possible to reduce the propagation of the dislocation from the seed crystal 25 to the group 13 nitride crystal 27. Therefore, the dislocation density of the group 13 nitride crystal 27 can be restricted to a small extent, and thereby the group 13 nitride crystals 80 to 83 can be more readily manufactured with higher quality and larger sized.

In the crystal manufacturing method according to the embodiment, the seed crystal 25 and the group 13 nitride crystal 27 grown from the seed crystal 25 may be of the same material (e.g. gallium nitride). Therefore, different from a case that hetero material such as aluminum nitride (AlN) is used as the seed crystal, the lattice constant and the thermal expansion coefficient can be matched and thereby the occurrence of the dislocation due to the difference in the lattice constant or thermal expansion coefficient can be suppressed. Therefore, a higher quality gallium nitride crystal can be produced than a gallium nitride crystal according to the prior art.

As aforementioned, according to the crystal manufacturing method of the embodiment, it is possible to manufacture the group 13 nitride crystals 80 to 83 of high quality with low dislocation density and of practical size.

In the above embodiment, the explanation was focused on the flux method for the crystal manufacturing method. However, the crystal manufacturing method is not limited to any particular one. For example, a vapor phase growth method such as HVPE, or any liquid phase growth method other than flux method may be used for the crystal growth.

[4] Group 13 Nitride Crystal

The group 13 nitride crystal according to the embodiment is any of group 13 nitride crystals 80 to 83 manufactured by the manufacturing method mentioned in [3]. Therefore, each of the group 13 nitride crystals 80 to 83 according to the embodiment includes, on the inner side thereof, at least a part of the gallium nitride crystal 25 mentioned in [2].

FIG. 8, FIG. 9, FIG. 10 and FIG. 11 schematically illustrate examples 80 to 83 of the group 13 nitride crystal according to the embodiment. As illustrated in drawings, any of the group 13 nitride crystals 80 (FIG. 8), 81 (FIG. 9), 82 (FIG. 10) and 83 (FIG. 11) includes therein the seed crystal 25.

It is sufficient that the seed crystal 25 is located inside of the group 13 nitride crystals 80 to 83. Thus, the seed crystal 25 may be located near the center (near the center of the hexagonal cross section) of the group 13 nitride crystals 80 to 83, as shown in FIG. 8, FIG. 9 and FIG. 10. Alternatively, the seed crystal 25 may be located near the periphery (in an area closer to edges of the hexagon than the center) of the group 13 nitride crystals 80 to 83, as shown in FIG. 11.

In an example of FIG. 8, for example, the group 13 nitride crystal 80 has a shape in which a six-sided pyramid-like shape is placed upon a hexagonal column-like shaped crystal having a top surface which becomes a bottom surface of the six-sided pyramid-like shape. However, shapes of the group 13 nitride crystals 80 to 83 are not limited to any particular shapes. For example, they may be of a six-sided pyramid-like shape without forming m-planes. In FIG. 9, there is illustrated the group 13 nitride crystal 81 having a shape in which a six-sided and truncated pyramid-like shape is placed upon a hexagonal column-like shaped crystal having a top surface which becomes a bottom surface of the six-sided and truncated pyramid-like shape. In FIG. 10, there is illustrated the group 13 nitride crystal 82 having a shape made of a six-sided and truncated pyramid-like shape and a hexagonal column-like shape, similarly to FIG. 9. In the crystal 82, a ratio of the crystal diameter to the length in the c-axis direction is different from the crystal 81. FIG. 11 shows a similar crystal shape to FIG. 10 except that the position of the seed crystal is different as mentioned before.

In the group 13 nitride crystals 80 to 83, additives may be doped in the group 13 nitride crystal 27. Examples of additives include a doping impurity such as germanium (Ge), oxygen (O), silicon (Si) and the like, an acceptor impurity such as magnesium (Mg), lithium (Li), strontium (Sr), calcium (Ca), zinc (Zn) and the like, a transition metal for providing magnetic property such as iron (Fe), manganese (Mn), chromium (Cr) and the like, a rare earth element or the like for providing magnetic property or fluorescence such as europium (Eu), erbium (Er), terbium (Tb), thulium (Tm) and the like, congeners such as aluminum (Al), indium (In) and the like. Other additives may de selected and doped depending on the purpose.

<Dislocation Density>

In the crystal manufacturing method according to the embodiment, the gallium nitride crystal 25 used as the seed crystal and the group 13 nitride crystal 27 grown from the seed crystal can be of the same gallium nitride. Different from a case that hetero material such as sapphire or aluminum nitride is used as the seed crystal, the lattice constant or thermal expansion coefficient can be matched, and thereby the occurrence of the dislocation due to the difference in the lattice constant or the thermal expansion coefficient can be suppressed. Furthermore, according to the embodiment, the second region 25 b having less impurities or defects is disposed on outer side in the cross section intersecting the c-axis of the gallium nitride crystal 25. Thereby, the high quality group 13 nitride crystal 27 can be manufactured when the group 13 nitride crystal 27 is grown from the gallium nitride crystal 25 as seed crystal. Namely, it is further possible to suppress the dislocation which may occur at a boundary surface of the gallium nitride crystal 25 used as seed crystal.

Next, an explanation will he made on the dislocation in the crystal with reference to FIG. 12. FIG. 12 schematically shows dislocations in a cross section parallel to the c-axis and the a-axis in the group 13 nitride crystal 80. Incidentally, FIG. 12 only shows an enlarged right part from the seed crystal 25 in a a-plane cross section of the group 13 nitride crystal 80. Although the following explanation will be focused on the group 13 nitride crystal 80, the same explanation will be applied to the group 13 nitride crystals 81 to 83.

In the embodiment, the group 13 nitride crystal 27 grown from the seed crystal 25 is grown mainly from m-planes comprising the outer periphery of the seed crystal 25 toward the m-axis direction (i.e. a direction in which the c-plane cross section of the hexagon enlarges). Therefore, the dislocations occurred at the growth boundary surface of the seed crystal 25 are likely to be oriented in <11-20> direction parallel to the crystal growth direction and are not likely to be oriented in <11-23> direction unparallel to the crystal growth direction.

With regard to the dislocation direction intersecting the c-plane of the group 13 nitride crystal having a hexagonal crystal structure, there are <0001> direction and <11-23> direction. In the embodiment, however, the dislocation n the <0001> direction does not occur and the dislocation in the <11-23> direction occurs quite a few.

Therefore, in a preferred embodiment, the dislocation density of the c-plane of the group 13 nitride crystal 27 (a region other than the seed crystal 25 in the group 13 nitride crystal 80) is smaller than that of the c-plane of the gallium nitride crystal 25 contained inside of the group 13 nitride crystal 27 (see Example 5).

[5] Manufacturing Method of Crystal Substrate

The manufacturing method of the crystal substrate according to the embodiment is a method of manufacturing the crystal substrate 100 from the group 13 nitride crystals 80 to 83 mentioned in [4].

FIG. 13 and FIG. 14 schematically show directions for slicing the group 13 nitride crystal 82 (FIG. 10) and the group 13 nitride crystal 83 (FIG. 11). FIGS. 15A to 15C, and FIGS. 16A to 16C schematically show examples of crystal substrate 100 (100 a to 100 f) obtained through slicing.

The manufacturing method according to the embodiment includes a process of cutting out the crystal substrate 100 so as to include at least a part of the seed crystal 25 when the group 13 nitride crystals 80 to 83 are sliced. For example, the crystal substrate 100 a as shown in FIG. 15A can be obtained by cutting or slicing the crystal 82 orthogonal to the c-axis of the seed crystal 25 as shown by a dotted-dashed line P1 in FIG. 13. Alternatively, the crystal substrate 100 b as shown in FIG. 15B can be obtained by cutting or slicing the crystal 82 oblique to the c-axis of the seed crystal 25 as shown by a dotted-dashed line P2 in FIG. 13. Alternatively, the crystal substrate 100 c as shown in FIG. 15C can be obtained by cutting or slicing the crystal 82 along the c-axis of the seed crystal 25 as shown by a dotted-dashed line P3 in FIG. 13.

The crystal substrate 100 as shown in FIGS. 15A to 15C and FIGS. 16A to 16C (100 a to 100 f) can be obtained through various treatments such as forming or shaping process, surface treatment and the like after slicing.

According to the manufacturing method of the embodiment, it is possible to obtain a large area of main surface of the substrate, since the crystal substrate 100 is cut out from the group 13 nitride crystals 80 to 83 elongated in the c-axis direction as mentioned above, whether the substrate is cut out along the c-plane or along a plane other than the c-plane. Namely, according to the embodiment, it is possible to produce the crystal substrate 100 having a large area and a main surface comprised of any crystal plane such as c-plane, m-plane, a-plane, {10-11} plane, {20-21} plane, {11-22} plane and so on. Therefore, it is possible to produce the crystal substrate 100 of practical size applicable to various semiconductor devices.

According to the manufacturing method of the embodiment, the crystal substrate 100 is produced by slicing the bulk crystal of the group 13 nitride (the group 13 nitride crystals 80 to 83). Different from the prior art, the method does not include a process of separating a thick crystal film from the hetero material substrate on which the crystal is grown and which has a thermal expansion coefficient and a lattice constant considerably different from that of the crystal thereon. Thereby, cracks hardly occur in the crystal substrate 100 according to the manufacturing method of the embodiment. Therefore, it is possible to produce the crystal substrate 100 of higher quality in comparison with the prior art.

[6] Crystal Substrate

The crystal substrate according to the embodiment is a crystal substrate 100 manufactured by the manufacturing method mentioned in [5]. Namely, the crystal substrate 100 of the embodiment characteristically includes at least a part of the seed crystal 25 mentioned in [2].

As shown in FIGS. 15A to 15C and FIGS. 16A to 16C, the seed crystal 25 used in the crystal manufacturing process mentioned in [3] is included in the crystal substrate 100 of the embodiments (100 a to 100 f). At least a part of the outer periphery comprised of m-planes of the seed crystal 25 is surrounded by the group 13 nitride crystal 27 grown from the seed crystal 25.

In a more preferred embodiment, an entire outer periphery comprised of m-planes of the seed crystal 25 is surrounded by the group 13 nitride crystal 27.

Insofar as the seed crystal 25 is included inside of the group 13 nitride crystal 27, the position thereof is not limited to any particular one. For example, as shown in FIG. 15A and FIG. 15B, the seed crystal 25 may be disposed around the center part of the main surface of the crystal substrate 100. In this case, the c-axis of the seed crystal 25 may be disposed perpendicular to the main surface as shown in FIG. 15A, or may be disposed oblique to the main surface as shown in FIG. 15B.

As shown in FIG. 15C and FIG. 16C, the seed crystal 25 may be disposed so that the c-axis of the seed crystal 25 is parallel to the main surface of the substrate. Furthermore, the seed crystal 25 may be disposed at a position other than the center area of the main surface of the substrate 100. For example, as shown in FIG. 16A and FIG. 16B, the seed crystal 25 may be disposed near the periphery of the main surface of the substrate 100.

In a preferred embodiment, the c-plane of the hexagonal crystal structure becomes a main surface of the crystal substrate 100.

As mentioned above, the dislocation extending through the c-plane (line defect) is likely to be reduced in the group 13 nitride crystal 27 according to a preferred embodiment. Therefore, when the crystal substrate 100 having a main surface comprised of c-plane is produced from the group 13 nitride crystal 80, the line defect extending in the c-axis direction can be reduced and a high quality crystal substrate 100 can be obtained.

[7] Preferred Shapes of Group 13 Nitride Crystal (Bulk Crystal)

Next, an explanation will be made on preferred shapes of the group 13 nitride crystals 80 to 83. Hereinafter, the group 13 nitride crystal BO will be explained with reference to FIG. 17 to FIG. 19. However, the same suitable shapes are applicable also to the group 13 nitride crystals 81 to 83. FIG. 17 to FIG. 19 schematically show examples of the crystal growth of the group 13 nitride crystal 27 from the seed crystal 25. In the following explanation, the crystal manufacturing method is not limited to any particular one. Incidentally, a m-plane cross section of the group 13 nitride crystal 80 is illustrated in FIG. 17 to FIG. 19, respectively.

As shown in FIG. 17, the group 13 nitride crystal 27 grown from the seed crystal 25 includes a region 27 a and a region 27 b. The region 27 a is grown mainly toward the m-axis direction (i.e. a direction in which the c-plane cross section of the hexagon enlarges) from the m-plane comprising the outer periphery of the seed crystal 25. The region 27 b is grown mainly from {10-11} plane of the seed crystal 25 or from {10-11} plane on the top of the region 27 .

In the region 27 b, the growth rate of forming {10-11} plane is considered to be the slowest or rate controlling. Thereby, the group 13 nitride crystal 27 grown around the upper part of the seed crystal 25 is considered to often become a six-sided pyramid-like shape.

FIG. 18 schematically shows an example of the crystal growth when the length L (FIG. 2) in the c-axis direction of the seed crystal 25 is short. In a case that the length L of the seed crystal 25 is not long enough, the ratio of the six-sided pyramid part to the hexagonal column-like part is large. Therefore, the region 27 b grown toward the <10-11> direction has a larger volume ratio to the region 27 a grown toward the m-axis direction. Therefore, the group 13 nitride crystal 80 is likely to be a shape illustrated in FIG. 18. In this case, the region 27 b is included in all the c-planes.

FIG. 19 shows a state that the group 13 nitride crystal 27 of FIG. 18 is further grown. As shown in FIG. 18, if the outer periphery of the seed crystal 25 is surrounded by the region 27 b, the outer periphery comprised of m-planes is not formed even if the crystal growth is continued. In this case, it is often observed that the group 13 nitride crystal 27 grows while maintaining the {10-11} plane as an outer peripheral surface.

The region 27 a is a region whose crystal growth is started from the outer periphery comprised of m-planes of the seed crystal 25. As mentioned relating to FIG. 12, the group 13 nitride crystal 27 (the region 27 a) grown mainly from the m-planes of the seed crystal 25 is considered to have relatively less dislocation extending through the c-axis direction. Therefore, when the crystal substrate 100 (e.g. 100 a, 100 b, 100 d, 100 e) whose main surface is a c-plane is produced, the region 27 a is preferably contained or included as much as possible.

In FIG. 17, if the crystal substrate 100 is produced by using a lower part of the group 13 nitride crystal 80, that is, a hexagonal column-like part without including the region 27 b, it is possible to obtain the crystal substrate 100 including no region 27 b considered to be grown from planes other than m-planes.

On the other hand, if the crystal substrate 100 is produced by using the group 13 nitride crystal 80 as shown in FIG. 18 or 19, the obtained substrate 100 includes both the region 27 a and the region 27 b. Generally, regions grown toward different directions often have different properties from each other. Furthermore, the seed crystal and the crystal grown from the seed crystal often have different properties. Therefore, the substrate 100 produced from the crystal 80 as shown in FIG. 18 or 19 has three regions, that is, the regions 27 a and 27 b, and the seed crystal 25. This may lower the quality of the substrate 100.

Therefore, in a preferred embodiment, the group 13 nitride crystal 80 is of a shape including a hexagonal column-like part at the lower part of the crystal. However, the _(p)referred shape is not limited to an example of FIG. 17. For example, the group 13 nitride crystal may grow from the seed crystal 25 mainly toward the m-axis direction and mainly have a hexagonal column-like shape.

[8] Preferred Size of Seed Crystal

Next, an explanation will be made on the shape of the seed crystal 25 suitable for manufacturing the group 13 nitride crystals 80 to 83 having the suitable shapes mentioned in [7]. The gallium nitride crystal 25 has a hexagonal crystal structure in which an angle formed between the “a+c”-axis (<11-23> direction) and the c-plane is 58.4 degrees. If the ratio L/d of the length L (FIG. 2) in the c-axis direction of the gallium nitride crystal 25 to the crystal diameter d in the c-plane cross section is 0.813, the gallium nitride crystal 25 becomes a six-sided pyramid-like shape.

As mentioned in [7], in order to obtain the good quality of the group 17 nitride crystals 80 to 83, the group 13 nitride crystal 27 is preferably grown mainly from the cuter surface comprised of m-planes of the seed crystal 25. In a preferred embodiment, the seed crystal 25 includes m-planes as the outer periphery.

FIG. 20 schematically shows various relationships between the ratio L/d and the shape of the seed crystal 25. As shown in FIG. 20, if the ratio L/d equals to 0.813 (L/d=0.813), the seed crystal 25 becomes a six-sided pyramid-like shape. If the ratio L/d is greater than 0.813 (L/d>0.813), the upper part becomes a six-sided pyramid-like shape and the lower part becomes a hexagonal column-like shape. And the outer periphery (side surfaces) of the seed crystal 25 includes m-planes. If the ratio L/d is smaller than 0.813 (L/d<0.813), the seed crystal 25 becomes a six-sided pyramid-like shape without including any m-plane, or becomes a shape having a truncated six-sided pyramid-like part with the c-plane formed on a top face and having a low profile hexagonal column-like part including the m-planes.

Therefore, in a preferred embodiment, the ratio L/d is greater than 0.813 in the seed crystal 25, in which L is the length in the c-axis direction and d is a crystal diameter in the c-plane.

The practical size of the crystal substrate 100 is preferably a half-inch (12.7 mm) or 2 inches (5.08 cm). In the following explanation, the explanation will be made on the size of the seed crystal 25 required in a case that group 13 nitride crystal substrate 100 having the c-plane as main surface has a diameter of half-inch (12.7 mm) or more, or a diameter of 2 inches or more.

In the following explanation, as an example of the minimum thickness required for the practical substrate, a case that the thickness of the crystal substrate 100 is 1 mm is simulated. However, the required minimum thickness is not limited to this, and can be simulated as appropriate.

In order for the diameter of the crystal substrate 100 to be 12.7 mm, that is, for the diameter d of the group 13 nitride crystals 80 to 83 to be 12.7 mm, the group 13 nitride crystal 27 is required to be grown to a radius direction (the m-axis direction) by 6.35 mm, if the diameter of the seed crystal 25 is neglected as zero.

For example, if the crystal growth rate Vm in the m-axis direction is assumed to be twice of the crystal growth rate Vc in the c-axis direction, the crystal is grown about 3.2 mm in the c-axis direction while grown 6.35 mm in the m-axis direction. As mentioned above, the ratio L/d is greater than 0.813 (L/d>0.813). Therefore, in order for the crystal diameter d (the diameter of the bottom surface of the hexagonal column-like part) to be 12.7 mm, the length L in the c-axis direction (the height of the six-sided pyramid-like part) is 11.9 mm. Therefore, the required length of the seed crystal 25 is calculated as “11.9−3.2=8.7” mm. That is, the minimum length of the seed crystal 25 required for obtaining the group 13 nitride crystal 80 having the six-sided pyramid-like shape as shown in FIG. 18 is 8.7 mm. It is desired that a hexagonal column-like region is formed under the six-sided pyramid-like part as shown in FIG. 17. If the required thickness of the crystal substrate 100 is assumed as 1 mm or more, the required length L in the c-axis direction of the seed crystal 25 is estimated as 9.7 mm.

Thus, in a preferred embodiment, the length L in the c-axis direction of the seed crystal 25 is preferably 9.7 mm or more.

In a more preferred embodiment, as for the seed crystal 25, the ratio L/d (the ratio of the length L in the c-axis direction to the diameter d in the c-plane) is preferably greater than 0.813, and the length L in the c-axis direction is preferably 9.7 mm or more. More preferably, L/d is greater than 7, and still more preferably, L/d is greater than 30.

As described above, according to the preferred embodiment, it is possible to produce the crystal substrate 100 having the diameter of the c-plane that is half-inch. As mentioned above, since the group 13 nitride crystals 80 to 83 grown from the m-plane of the seed crystal 25 is of high quality, a large and high quality crystal substrate 100 can be produced.

In order to obtain the crystal substrate 100 having the diameter of 2 inches (5.08 cm), the required length L in the c-axis direction of the seed crystal 25 is estimated as 37.4 mm or more.

Therefore, in a preferred embodiment, the length L in the c-axis direction of the seed crystal 25 is preferably 37.4 mm or more. Thereby, it is possible to produce the crystal substrate 100 having the diameter in the c-plane that is 2 inches. As mentioned above, since the group 13 nitride crystals 80 to 83 grown from the m-plane of the seed crystal 25 are of high quality, a large diameter and high quality crystal substrate 100 of the gallium nitride can be produced.

EXAMPLES

Examples will be shown below for further understanding the invention. However, the invention is not limited to these examples. Incidentally, reference numerals correspond to configurations of crystal manufacturing apparatuses explained with reference to FIG. 1 and FIG. 7.

Example 1 Manufacturing Example 1 of Seed Crystal

In this Example, the gallium nitride crystal 25 was manufactured by using the crystal manufacturing apparatus 1 illustrated in FIG. 1. In this Example, as an example of the method (1), gallium having the nominal purity 99.99999% and sodium having the nominal purity of 99.95% were input with a mol ratio as 0.25:0.75 into the reactor vessel 12 made of a sintered BN and having the inner diameter of 92 mm.

In a glove box, the reactor vessel 12 was placed into the inner vessel 11 under a high purity Ar gas atmosphere. The valve 31 was closed to shut out the inner space of the reactor vessel 12 from the outer atmosphere, so that the inner vessel 11 was sealed under the state filled with Ar gas. Then, the inner vessel 11 was taken out from the glove box and then assembled into the crystal manufacturing apparatus 1. Specifically, the inner vessel 11 was installed at a predetermined position with respect to the heater 13 and connected to the gas supplying pipe 14 of nitrogen gas and argon gas at the valve 31.

Next, after argon gas was purged from the inner vessel 11, the nitrogen gas was introduced from the nitrogen supplying pipe 17. The nitrogen gas was subjected to the pressure control by the pressure regulator 16, and the valve 15 was opened, so that the nitrogen pressure in the inner vessel 11 became 3.2 MPa. Then, the valve 15 was closed and the pressure regulator 16 was set at 8 MPa. Then, the heater 13 was turned on to heat the reactor vessel 12 to a crystal growth temperature. In this Example, the crystal growth temperature was 870 degrees Celsius.

At the crystal growth temperature, gallium and sodium in the reactor vessel 12 were melted to form a molten mixture 24. The temperature of the molten mixture 24 was the same as the temperature of the reactor vessel 12. Until heated to this temperature, in the crystal manufacturing apparatus 1 of this Example, gas in the inner vessel 11 was heated so that the whole pressure became 8 MPa.

Next, the valve 15 was opened so that the pressure of nitrogen gas became 8 MPa, and the pressure equilibrium state was established between the inside of the inner vessel 11 and the inside of the nitrogen supplying pipe 17.

After a crystal of gallium nitride was grown by maintaining the reactor vessel 12 for 500 hours under this state, the heater 13 was controlled to cool the inner vessel 11 to a room temperature (around 20 degrees Celsius). After decreasing the gas pressure in the inner vessel 11, the inner vessel 11 was opened. In the reactor vessel 12, many gallium nitride crystals 25 were formed. Grown gallium nitride crystals 25 were colorless and transparent, had the crystal diameter d of approximately 100 to 1500 μm, had the length L of approximately 10 to 40 mm, and had the ratio L/d of the length L to the crystal diameter d was approximately 20 to 300. The gallium nitride crystals 25 were grown generally parallel to the c-axis, and had the m-planes (see FIG. 3) formed on the side surfaces of each crystal.

Example 2 Manufacturing Example 2 of Seed Crystal

In Example 2, as an example of the manufacturing method mentioned in (2), a crystal growth was conducted similarly to Example 1 except that a reactor vessel 12 made of YAG was used, a plate of sintered BN was fit in the bottom of the reactor vessel 12, and the crystal growth time was 300 hours.

As a result, similarly to Example 1, many colorless and transparent gallium nitride crystals 25 were formed. The crystal diameter d was approximately 100 to 600 μm, the length L was approximately 10 to 15 mm, and the ratio L/d of the length L to the crystal diameter d was approximately 30 to 500.

Comparative Example 1

A crystal growth was conducted similarly to Example 1 except that the crystal growth temperature was 860 degrees Celsius, and the nitrogen partial pressure was 5 MPa. As a result, although gallium nitride microcrystals of approximately several hundred micrometer size were obtained, any gallium nitride single crystal having a length of 3 mm or more was not obtained.

The gallium nitride crystals 25 manufactured in Examples 1 and 2, and Comparative Example 1 were subject to various measurements. The measurement results were shown below.

<Photoluminescence (PL) Measurement Result>

Photoluminescence (PL) was measured at a room temperature (25 degrees Celsius) with respect to the gallium nitride crystals manufactured in Examples 1 and 2. Photoluminescence was measured by using LabRAM HR-800 fabricated by HORIBA, Ltd. As an exciting light source, helium-cadmium (He—Cd) laser having a wavelength of 325 nm was used. Photoluminescence was measured with respect to the first region 25 a which was an inner region of the seed crystal 25 and with respect to the second region 25 b which was an outer region of the seed crystal 25, respectively.

FIG. 5 shows one example of measurement result of emission spectra of PL with respect to the first region 25 a and the second region 25 b illustrated in FIG. 4. The horizontal axis means wavelength (nm), and the vertical axis means emission intensity.

As shown by a solid line in FIG. 5, as for the first region 25 a, a broad emission (the second peak) having a peak around 600 nm was observed in a range of 500 nm to 800 nm, while only a very weak intensity of emission was observed for an emission (the first peak) from a vicinity of band edge (365 nm) of gallium nitride.

On the other hand, as shown by a dotted line in FIG. 5, as for the second region 25 b, a strong peak intensity of emission (the first peak) from the vicinity of band edge (365 nm) of gallium nitride was observed, while only a very weak intensity of emission was observed for a broad emission (the second peak) in a range of 500 nm to 800 nm.

Thus, with respect to the seed crystals 25 manufactured by Examples 1 and 2, it was confirmed that the peak intensity of the first peak was smaller than the peak intensity of the second peak in the first region 25 a contained on inner side of the seed crystal 25. It was also confirmed that the peak intensity of the first peak was greater than the peak intensity of the second peak in the second region 25 b located on outer side of the seed crystal 25.

Next, with reference to FIG. 21 and FIG. 22, an explanation will be made on emission intensity distributions of photoluminescence. FIG. 21 and FIG. 22 show one example of photoluminescence measurement result which was measured as for the c-plane cross section of the gallium nitride crystals 25 manufactured by Examples 1 and 2, showing spectral intensity with different wavelength band at the same measurement point on the c-plane cross section.

FIG. 21 shows a mapping image of the spectral intensity in a range of 360 nm to 370 nm of photoluminescence. Darker color indicates stronger spectral intensity in a range of 360 nm to 370 nm.

FIG. 22 shows a mapping image of the spectral intensity in a range of 500 nm to 800 nm of photoluminescence. Darker color indicates stronger spectral intensity in a range of 500 nm to 800 nm.

Therefore, according to the mapping result of FIG. 21 and FIG. 22, it was confirmed that the gallium nitride crystal 25 had the first region 25 a on inner side thereof and the second region 25 b on outer side thereof.

Also, as a result of PL measurement as for the c-plane cross section of the seed crystal 25 manufactured by Examples 1 and 2, it was confirmed that the second region 25 b surrounds a whole outer periphery of the first region 25 a in some gallium nitride crystals 25, as shown in FIG. 4. In other gallium nitride crystals 25, it was confirmed that the second region 25 b surrounds a part of the outer periphery of the first region 25 a, as shown in FIG. 3. Thus, it was confirmed that the second region 25 b surrounds at least a part of the outer periphery of the first region 25 a, in the c-plane cross section of the gallium nitride crystals 25 manufactured by Examples 1 and 2.

On the other hand, PL measurement of the cross section of gallium nitride microcrystals manufactured by Comparative Example 1 was conducted. As a result, in the cross section of microcrystals, only the second region 25 b in which the peak intensity of the first peak was greater than the peak intensity of the second peak was confirmed and the first region 25 a was not confirmed.

According to the above results, it was confirmed that the gallium nitride crystal 25 had a double layer structure including the first region 25 a and the second region 25 b, in a case that the reactor vessel 12 made of sintered BN was used (Example 1) and a case that the plate of sintered BN was placed in the reactor vessel 12 (Example 2). On the other hand, in the case of Comparative Example 1, the first region 25 a was not confirmed, and it was confirmed that the crystal did not have the double layer structure including the first region 25 a and the second region 25 b. Therefore, it was confirmed that the seed crystal 25 having both the first region 25 a and the second region 25 b can be produced according to the methods of Examples 1 and 2.

<Boron Density Measurement>

Boron density in the crystal was measured by using a secondary ion mass spectrometer (SIMS) with respect to the gallium nitride crystals 25 manufactured by Examples 1 and 2. As SIMS, IMS 7f (model name) fabricated by CAMECA was used. As primary ion beam, Cs⁺ ion was used. The primary ion accelerating voltage was 15.0 kV, and the detection area was 30 μmΦ. In this measurement, the boron density was measured at a plurality of points with respect to the inner region (i.e. the first region 25 a) and with respect to the outer region (i.e. the second region 25 b) in the c-plane cross section of the gallium nitride crystal 25, respectively, as shown in FIG. 4.

Although there were some variations depending on the measurement points, the boron density in the first region 25 a was approximately 5×10¹⁸ cm⁻³ to 3×10¹⁹ cm⁻³, and the boron density in the second region 25 b was approximately 1×10¹⁶ cm⁻³ to 8×10¹⁷ cm⁻³.

Thus, with respect to the gallium nitride crystals 25 manufactured by Examples 1 and 2, it was confirmed that the boron density in the second region 25 b on outer side was lower than that of the first region 25 a on inner side in the c-plane cross section. Thus, it was confirmed that there was a double layered structure made of the first region 25 a and the second region 25 b.

On the other hand, the c-plane cross section of the gallium nitride microcrystal manufactured by Comparative Example 1 did not have a layered structure with different boron densities.

According to the above results, the first region 25 a having a relatively higher boron density and the second region 25 b having a boron density lower than that of the first region 25 a were confirmed, in a case that the reactor vessel 12 made of sintered BN was used (Example 1) and a case that the plate of sintered BN was placed in the reactor vessel 12 (Example 2). On the other hand, it was confirmed that the double layered structure was not formed in the case that the crystal growth was conducted under conditions that the crystal growth temperature was 860 degrees Celsius and the nitrogen partial pressure was 5 MPa (Comparative Example 1).

Next, an explanation will be made on an Example in which the group 13 nitride crystal 80 was manufactured by conducting a seed growth (SG) of the seed crystal 25 according to the crystal manufacturing method mentioned in [3].

Example 4 Growing Example of Bulk Crystal from Seed Crystal

In this Example, as an example of the group 13 nitride crystal 80, gallium nitride crystal 80 was manufactured by growing the seed crystal 25 in the crystal manufacturing apparatus 2 as illustrated in FIG. 7. As the seed crystal 25, the gallium nitride crystal manufactured by Example 1 having a width of 1 mm and a length of 40 mm was used.

Firstly, the inner vessel 51 was separated from the crystal manufacturing apparatus 2 at the valve 61 portion, and placed into the glove box under Ar atmosphere. Next, the seed crystal 25 was placed in the reactor vessel 52 made of alumina and having an inner diameter of 140 mm and a depth of 100 mm. The seed crystal 25 was retained by inserting the crystal 25 into a hole having a depth of 4 mm made at the bottom of the reactor vessel 52.

Next, sodium (Na) was heated to be liquid and then put into the reactor vessel 52. After the sodium was solidified, gallium was input. In this Example, a mol ratio of gallium to sodium was 0.25:0.75.

Then, in the glove box, the reactor vessel 52 was placed into the inner vessel 51 under a high purity Ar gas atmosphere. Then, the valve 61 was closed to seal the inner vessel 51 filled with Ar gas, and shut out the inner space of the reactor vessel 52 from the outside atmosphere. Next, the inner vessel 51 was taken out from the glove box and then assembled into the crystal manufacturing apparatus 2. Specifically, the inner vessel 51 was installed at a predetermined place with respect to the heater 53, and connected to the gas supplying pipe 54 at the valve 61 portion.

Next, after argon gas was purged from the inner vessel 51, nitrogen gas was introduced from the nitrogen supplying pipe 57. The nitrogen gas was subjected to the pressure control by the pressure regulator 56, and the valve 55 was opened, so that the whole pressure in the inner vessel 51 was 1.2 MPa. Then, the valve 55 was closed and the pressure regulator 56 was set at 3 MPa.

Next, the heater 53 was turned on to heat the reactor vessel 52 to a crystal growth temperature. In this Example, the crystal growth temperature was 870 degrees Celsius. Then, similarly to Example 1, the valve 55 was opened so that the nitrogen gas pressure became 3 MPa. Gallium nitride crystal 27 was formed by maintaining the reactor vessel 52 for 1300 hours under this state.

As a result, in the reactor vessel 52, a gallium nitride crystal 80 (single crystal) having a larger crystal diameter increased in a direction orthogonal to the c-axis was formed from the gallium nitride crystal 25 as the seed crystal. The obtained gallium nitride crystal 80 was generally colorless and transparent, and had the crystal diameter d of 51 mm, the length L in the c-axis direction of 54 mm including the seed crystal part 25 which were inserted into the hole at the bottom of the reactor vessel 52, The gallium nitride crystal 80 had a shape in which the upper part was a six-sided pyramid-like shape and the lower part was a hexagonal column-like shape, as shown in FIG. 17.

The property of the c-plane cross section of the gallium nitride crystal 80 obtained in Example 4 will be described in the following Example 5.

Example 5 Manufacturing Example of C-Plane Substrate

A gallium nitride crystal substrate 100 (see FIG. 15A) having c-plane as main surface and having an outer diameter (Φ) of 2 inches and a thickness of 400 μm was produced by grinding the outline of the gallium nitride crystal 80 manufactured by Example 4, slicing the grinded crystal parallel to the c-plane, polishing the sliced crystal surface(s), and treating surface(s) of the polished crystal.

<PL Measurement>

Next, photoluminescence of a c-plane of the crystal substrate 100 was measured at a room temperature. As an exciting light source, He—Cd laser having a wavelength of 325 nm was used.

FIG. 23 schematically shows photoluminescence emission distribution in the crystal substrate 100 of Example 5. The seed crystal 25 had regions 251 to 253 which have different emission properties such as color and intensity of photoluminescence from each other. Difference in emission intensity or color among regions is considered because impurities or amount thereof are different among regions.

The region 251 is a region exhibiting a strong red emission. According to the red emission, it can be understood that an emission from 600 to 650 nm which is an emission other than the vicinity of the band edge of gallium nitride is strong. Namely, the region 251 is a region having the property of the first region 25 a.

The region 252 is a region exhibiting a strong blue emission. According to the blue emission, it can be understood that it is an emission from the band edge of gallium nitride or the vicinity thereof. The region 253 is a region where the blue emission intensity is smaller than that of the region 252. In the region 252 or 253, the orange or red emission is not observed, and an emission other than the vicinity of the band edge of gallium is not strong. Namely, the region 252 is a region having the property of the second region 25 b 1 and the region 253 is a region having the property of the second region 25 b 2. In the c-plane cross section of this seed crystal 25, the thickness t of the second region 25 b (thickness in the m-axis direction, see FIG. 2) was confirmed to be at least 10 μm or more.

Incidentally, the gallium nitride crystal 27 grown around the seed crystal 25 exhibited a blue emission.

Thus, according to the PL measurement results, it was confirmed that the c-plane of the crystal substrate 100 includes the first region 25 a, the second region 25 b 1 and the second region 25 b 2. Also, it was confirmed that the first region 25 a was surrounded by the second region 25 b 1, and the second region 25 b 1 was surrounded by the second region 25 b 2.

<Dislocation Density Measurement>

The dislocation density was obtained in such a manner that the c-plane surface of the crystal substrate 100 was etched by acid (mixed acid of phosphoric acid and sulfuric acid, 230 degrees Celsius) to obtain the etch pit density. The obtained etch pit density was evaluated as the dislocation density. As a result, the dislocation density of the seed crystal 25 was 6×10⁷ cm⁻² or less. The dislocation density of the gallium nitride crystal 27 grown from the seed crystal 25 was in the order of 10² cm⁻². According to the microscopic observation, it was confirmed that the crystal substrate 100 had no crack.

Thus, it was confirmed that the gallium nitride crystal 27 grown from the seed crystal 25 having the first region 25 a and the second regions 25 b 1, 25 b 2 exhibited the dislocation density smaller than 10⁶ cm⁻² and had no crack, and thereby the obtained crystal 27 was of high quality crystal.

The outermost periphery of the seed crystal 25 is not necessarily surrounded by the second region 25 b 1 (e.g. the region 252) exhibiting the strong blue emission. Even in a case that the outermost periphery of the seed crystal 25 is surrounded by the second region 25 b 2 (e.g. the region 253) where the blue emission is weak and the emission other than the vicinity of the band edge of gallium nitride is not strong, it was confirmed that the gallium nitride 27 grown from the second region 25 b 2 became high quality.

Example 6 Manufacturing Example of Crystal Substrate

A gallium nitride crystal substrate 100 (see FIG. 18) having a-plane as main surface and having a height of 40 mm, a width of 40 mm and a thickness of 400 μm was produced by grinding the outline of the gallium nitride crystal 80 manufactured by Example 4, slicing the grinded crystal parallel to the a-plane, polishing the sliced crystal surface(s), and treating surface(s) of the polished crystal. Furthermore, the crystal substrate 100 having m-plane as main surface, and the crystal substrate 100 having {10-11} plane as main surface were produced.

Each main surface of the crystal substrates 100 each having a-plane main surface, m-plane main surface, and (10-11) plane main surface was etched by acidic solution (mixed acid of phosphoric acid and sulfuric acid, 230 degrees Celsius) to obtain the etch pit density. The obtained etch pit density was evaluated as the dislocation density. As a result, the dislocation density of any substrate was in the order of 10⁶ cm⁻² to 10⁷ cm⁻². Therefore, it was confirmed that these substrates had the dislocation density greater than that of the c-plane crystal substrate 100 mentioned in Example 5. According to the microscopic observation, it was confirmed that the crystal substrate 100 had no crack.

Thus, according to Examples 5 and 6, it was confirmed that the crystal substrate having smaller dislocation density can be produced by processing it so that the c-plane became the main surface.

As explained above, it was confirmed that a high quality c-plane crystal substrate can be produced with low dislocation density and no crack according to Examples.

According to the present invention, at least a part of the outer periphery of the cross section intersecting the c-axis of the gallium nitride crystal is the second region where the peak intensity of the first peak including the band edge emission of gallium nitride is larger than the peak intensity of the second peak located in longer wavelength area than the first peak. When the crystal quality is good, the peak intensity of the first peak including the band edge emission of gallium nitride is larger than the peak intensity of the second peak located in longer wavelength area than the first peak. Therefore, it means that at least a part of the periphery of the aforementioned cross section has good crystallinity. Therefore, when this gallium nitride crystal is used as the seed crystal to grow the group 13 nitride crystal, a high quality group 13 nitride crystal with low dislocation density can be produced. This group 13 nitride crystal can be used to produce a high quality crystal substrate.

Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth. 

1. A gallium nitride crystal having a hexagonal crystal structure comprising: a first region located on an inner side of a cross section intersecting c-axis of the hexagonal crystal structure; and a second region surrounding at least a part of the outer periphery of the first region in the cross section, wherein an emission spectrum of each of the first region and the second region with electron beam or ultraviolet light excitation has a first peak including a band edge emission of gallium nitride and a second peak located in a longer wavelength area than the first peak, a peak intensity of the first peak is smaller than a peak intensity of the second peak in the first region, and a peak intensity of the first peak is greater than a peak intensity of the second peak in the second region.
 2. The gallium nitride crystal according to claim 1, wherein the emission spectrum is measured at a room temperature, and the second peak is located in a wavelength range of 450 nm to 650 nm.
 3. The gallium nitride crystal according to claim 2, wherein the second peak is located in a wavelength range of 590 nm to 650 nm.
 4. The gallium nitride crystal according to claim 1, wherein the second region surrounds an entire outer periphery of the first region in the cross section.
 5. The gallium nitride crystal according to claim 1, wherein a thickness of the second region is 100 nm or more in the cross section.
 6. The gallium nitride crystal according to claim 1, wherein a boron density of the first region is higher than the boron density of the second region.
 7. The gallium nitride crystal according to claim 6, wherein the boron density of the first region is 4×10¹⁸ atms/cm³ or more, and the boron density of the second region is less than 4×10¹⁸ atms/cm³.
 8. The gallium nitride crystal according to claim 1, wherein a length L in a c-axis direction is 9.7 mm or more, and a ratio L/d which is a ratio of the length L to a crystal diameter d in c-plane is larger than 0.813.
 9. The gallium nitride crystal according to claim 8, wherein the length L is 37.4 mm or more.
 10. A group 13 nitride crystal containing therein at least a part of the gallium nitride crystal according to claim
 1. 11. The group 13 nitride crystal according to claim 10, wherein a dislocation density of a region other than the gallium nitride crystal in c-plane of the group 13 nitride crystal is smaller than the dislocation density of the gallium nitride crystal contained in the c-plane on an inner side of the group 13 nitride crystal.
 12. A group 13 nitride crystal substrate containing at least a part of the gallium nitride crystal according to claim
 1. 13. The group 13 nitride crystal substrate according to claim 12, wherein a c-plane of the hexagonal crystal structure is a main surface.
 14. A method of manufacturing a gallium nitride crystal, the method comprising: a first process to grow a first region of the gallium nitride crystal containing boron; and a second process to grow a second region of the gallium nitride crystal outside of the first region, the second region having a boron density lower than the boron density of the first region.
 15. A method of manufacturing a group 13 nitride crystal, wherein the gallium nitride crystal according to claim 1 is used as a seed crystal to grow the group 13 nitride crystal. 